removing and appending elemets from and to a list

Question

I have a list with ~ 500 model objects. The names of this objects are v1:

existing.list <- vector("list", 3)
v1 <- names(existing.list) <- c("A", "B", "C")

I now get different dataset, which i need to model, too, and save in the same list. The objects in this new dataset are overlapping with the some of the objects in existing.list. Because it is very time-consuming, i do want to keep the old results. The names of this new dataset are v2:

v2 <- c("B", "C", "D")

I first want to remove the objects in v1, which are not in v2, then append to existing.list all the new, unique names from v2. I can do the first task in a rather complicated way:

rm <- v1[!v1 %in% v2]
rm.i <- which(v1 %in% rm)
v1 <- v1[-rm.i]

But then i fail at appending the new objects, as determined by the unique elements in v2:

new.elements <- v2[!v2 %in% v1]

The desired output is a modified existing.list, with intact elements "B" and "C" and a new empty element "D". Basically, its a list with elements determined by the names in v2, but for a number of reasons it would be complicated to just create a new list and copy parts of existing.list to it. Since i need to do this for a number of lists, a less complicated way than i am doing now would be handy.

Thank you very much in advance! This is a last minute addition to a project, so any help is highly appreciated!

this question is based on a previous question, which i sloppily worded and thus created confusion. My thanks to those users, who still tried to help me.

Answer 1

If I understood you correctly you can first get the names of elements that are in v2 but not in v1

tmp <- setdiff(v2, v1) # "D"

And then subset existing.list and append it as follows

existing.list <- c(existing.list[v1 %in% v2], 
                   setNames(vector("list", length(tmp)), tmp))

Result

existing.list
#$B
#NULL

#$C
#NULL

#$D
#NULL

Answer 2

Are these what you are looking for?

intersect(v1, v2)
# [1] "B" "C"

setdiff(v2, v1)
# [1] "D"